Process solutions containing nickel and/or copper frequently contain selenium. Selenium is an undesirable impurity in copper since it decreases ductility and conductivity. Selenium is also undesirable in nickel for metallurgical reasons; hence, chemical specifications, particularly for melting grade nickel, severely limit the selenium content. Process solutions contemplated for treatment in accordance with the invention usually result from leaching processes wherein ores, mattes, etc. are leached with sulfuric acid.
Selenium present as Se (IV) in process solutions is readily removable therefrom at ambient temperatures and pressures using reductants such as sulfur dioxide, hydrogen, metal sulfides or metal powders such as nickel or iron powders. However, when selenium is present as Se (VI), a much more difficult problem is presented. Thus, it becomes necessary with the aforementioned reductants to resort to temperatures exceeding the boiling point of the solution; i.e., at least 110.degree. C. and superatmospheric pressures of at least 10 psig. Resort to such temperatures and pressures greatly increases capital and operating costs in conducting the process and also greatly increases operating difficulties and tends to reduce throughput as the required autoclaves are generally operated in a batch-type manner. Safety, throughput and cost savings greatly favor operating at ambient temperatures and pressures. However, efficient Se (VI) removal is still a requirement.